1. Field of the Invention
This invention relates to cardiac pacemakers and, more particularly, to dual chamber pacemakers and methods of pacing which optimize pacemaker response to early sensed atrial beats.
2. Description of the Prior Art
Various types of pacemakers are disclosed in the prior art, and are presently in widespread use. The pacing literature has documented the different types of pacemakers and their characteristics extensively. An excellent summary of the evolution and characteristics of pacemaker types, and specifically different types of dual chamber pacemakers, is set forth in U.S. Pat. No. 4,951,667, Markowitz et al., which is incorporated herein by reference.
Another and more recent advance in the field of cardiac pacing systems is that of the rate responsive pacemaker which increases cardiac output in response to exercise or other body demands. Such pacemakers may control pacing rate based upon sensing any one or a combination of different body parameters such as body activity, blood pH, respiratory rate, QT interval or historical atrial activity. See, for example, U.S. Pat. No. 4,428,378, Anderson et al., disclosing a pacemaker which varies pacing rate in response to sensed patient activity; and U.S. Pat. No. 4,228,308, Rickards, which discloses controlling pacing rate in response to Q-T interval. Additionally, rate responsive control has been integrated into dual chamber pacing systems, e.g., DDDR and DDIR systems. See "Rate Responsive Dual Chamber Pacing" in PACE, vol. 9, pp. 987-991; U.S. Pat. No. 4,467,807, Bornzin; and the above-noted U.S. Pat. No. 4,951,667.
A problem that has been recognized as arising in dual chamber pacemakers is that of "competitive atrial pacing" where, following an early natural atrial depolarization, the pacemaker may either fail to achieve atrial capture with a subsequent atrial pace pulse, or may induce an arrhythmia due to delivery of the atrial pacing pulse too closely following the atrial depolarization. Dual chamber pacing at rapid rates using prior art pacemakers may result in pacing the atrium near and possibly within the atrial cardiac refractory period of PACs and retrograde P waves. If atrial pacing occurs within the natural refractory period of the atrium, i.e., the refractory period of the heart, not that of the pacemaker, the pace stimulus will not capture the atrium. In such a situation, the actual interval between atrial and ventricular depolarizations may be prolonged beyond the physician-programmed AV interval. Further, if atrial stimuli are delivered such that they fall not in the natural refractory period of the atrium, but immediately afterwards, there exists the potential to initiate atrial fibrillation, atrial flutter, or other re-entrant tachycardias. This likelihood is especially prevalent in patients with a prior history of atrial arrhythmias.
One prior art response to the detection of a PAC is to switch the mode of the dual chamber pacemaker into a mode of operation not synchronized to the atrium, i.e., sacrifice atrial synchronous operation and deliver only ventricular pacing pulses until natural atrial signals are sensed having timing which enables switching back to atrial synchronous operation. See, for example, the patent to Funke et al., U.S. Pat. No. 5,027,815, also incorporated herein by reference, where the response to an early atrial sense is to simply inhibit delivery of an atrial pulse and proceed to deliver the ventricular pulse at the scheduled time. In this type of dual chamber system, ventricular rate regularity is maintained, but at the expense of AV synchrony.
Switching to a mode of operation in which ventricular pacing is not synchronized to the atrium or pacing the atrium at a time which causes loss of atrial capture with a resultant long AV interval, has the adverse effect of hemodynamic loss of the atrial kick on ventricular filling, and decrease in subsequent cardiac output. Also, a long interval between atrial and ventricular depolarizations allows the normal AV conduction system to repolarize and conduct retrogradely to the atrium. In this manner, the patient may suffer from the contraction of the atria on closed AV valves (i.e., the pacemaker syndrome) and even worse, the patient may be subject to initiation of pacemaker-mediated tachycardia (PMT). Maintaining short or reasonable AV intervals is critical to prevention of PMT.
A response to these concerns is found in the patent to Markowitz et al., U.S. Pat. No. 4,951,667, incorporated herein by reference. The pacemaker of this patent responds to an atrial sense (AS) that is deemed early by delaying the timing of the atrial pulse (AP) which is to be delivered. The scheduled VA interval is normally set by a sensor, i.e., it is a rate responsive interval. When and if a natural atrial signal is sensed within a relative atrial refractory period following a paced or sensed ventricular depolarization, and also within a safety period just preceding the time out of the scheduled VA escape interval, then the VA interval is shifted to a longer interval corresponding to a pre-set lower rate instead of the sensor determined V-A interval. Thus, instead of delivering an atrial pulse at the normal scheduled time, it is delayed following such an early atrial sense, and the ventricular pace pulse (VP) is synchronized to the delayed atrial pace pulse. The resulting beat to beat variability of the pacing rate may be substantial.
There thus remains a need for a pacemaker, and method of pacing which maintains safe, synchronized dual chamber pacing wherever possible and maintains a regular ventricular paced rhythm. It is undesirable from both hemodynamic and electrophysiologic rationales to allow atrial pacing during times at which the pacemaker may either fail to capture or may induce an arrhythmia. It is also undesirable to allow large beat-to-beat variations in ventricular paced rhythm. It is the intention of the atrial sync pace feature of this invention to reduce the occurrences of such behavior, and to deliver safe atrial pulses in a timed relation to subsequent ventricular pulses so as to maintain an appropriate ventricular pacing rate and to maintain AV synchrony whenever possible.